1. Field of the Invention
The invention relates to steam generator heating tube repair sleeves and methods for repairing steam generator heating tubes, and in particular to steam generators used in nuclear power plants.
2. Description of the Related Art
It is a matter of common knowledge that steam generators are used, inter alia, in power stations, especially in nuclear power stations. Steam generators are heat exchangers, through which a heated primary medium flows in a first circuit during operation, wherein the heat is then transferred to a secondary medium, in particular water, which flows in a second circuit separate from the first. After flowing through the steam generator, the water has then changed to a gaseous state of aggregation and is then available to drive a power station turbine. Especially in the case of nuclear power stations, where the primary medium is radioactively contaminated, strict and absolutely reliable separation between the first and second circuits in the steam generator must be preserved.
Steam generators essentially have a generally cylindrical vessel, which has respective clad tube plates at the axial ends thereof, wherein the interior space formed in this way is traversed by a multiplicity of steam generator heating tubes, which are joined to the cladding. In this way, as large as possible a contact area for heat exchange is formed between the two cooling circuits.
Steam generator heating tubes are welded leak-tightly to the cladding on the lower edge of the tube plate. If this weld seam is damaged, e.g. by foreign bodies, the leaktightness of the seam is no longer assured, and there may be a leak between the primary and the secondary medium. The damaged weld seams must be repaired in order to re-establish the separation between the primary and secondary media.
Hitherto, this has been accomplished by installing welded plugs. In this case, the standard practice is for steam generator heating tubes that have a leak at the weld seam to be sealed by means of welded plugs. To install welded plugs, a specific milled contour is first of all introduced. The bottom flange of the welded plug is used as filler material and is welded to the cladding of the tube plate by means of a TIG method (Tungsten Inert Gas). The seam serves both as a load bearing seam and as a sealing seam.
The disadvantage here is that the sealed steam generator heating tube no longer contributes to heat exchange between the primary and secondary circuits, and therefore there is a reduction in the efficiency of the power station. Because the steam generator also has a safety function in the case of incidents or accidents, the proportion of sealed tubes must not exceed a certain value, e.g. 10%. If the number of damaged weld seams in a steam generator is very high, installing welded plugs is no longer suitable as a repair measure.
Another prior art method involves re-welding. By re-melting the material, it is possible to close up defects in the seam, such as cracks or pores. However, in the case of re-welding, only small and superficial defects can be eliminated. Because no filler material is introduced, it is not possible to compensate for a lack of welding material. Moreover, the impurities in the original weld seam may be melted in again during re-welding, thereby possibly impairing the quality of the new seam. Re-welding damaged weld seams is therefore not a reliable and long-term repair method.
In addition, repair methods that involve repair sleeves are known. Thus, a repair sleeve for insertion into a defective tube end of a steam generator is disclosed in U.S. Pat. No. 4,592,577. A repair sleeve for insertion into a defective tube end of a steam generator is likewise disclosed in GB 1141239, wherein subsequent expansion by means of a rolling process is performed, giving a leak-tight joint with the tube end.